Sustainable treatment systems for removal of pharmaceutical residues and other priority persistent substances

Pharmaceutical residues and other emerging substances commonly summarised as micropollutants pass through wastewater treatment plants (WWTPs) and end up in the receiving waters and sludge. Many studies have investigated the removal efficiency of various techniques but a holistic evaluation of various relevant treatment alternatives regarding both the removal efficiency for various micropollutants, investment and operating costs, environmental impacts and future comprehensiveness is still lacking. This paper provides the results from a large 3-year project about the evaluation of sustainable treatment systems for removal of various micropollutants or disruptive effects at Swedish WWTPs and their environmental, economic and future sustainability. The presented results are based on our own pilot tests and related assessment and modelling efforts and provide a holistic view on advanced treatment of wastewater for removal of micropollutants.

Bacterial methylation of chlorinated phenols and guaiacols: formation of veratroles from guaiacols and high-molecular-weight chlorinated lignin

Two strains of bacteria, provisionally assigned to the genus Arthrobacter, were shown to metabolize mono-, di-, tri-, and tetrachloroguaiacols and pentachlorophenol to the corresponding O-methyl compounds. Hydroxylated intermediates were formed only transiently, except for the synthesis by one strain of 3,4,5-trichlorosyringol from 3,4,5-trichloroguaiacol. Two isomeric trichloroveratroles and tetrachloroveratrole were formed by three of the strains from a high-molecular-weight chlorinated lignin isolated from kraft pulp mill bleach plant. The concentrations of methylated metabolites varied widely and did not appear to be correlated with degradation. The possible environmental consequences resulting from synthesis of these highly lipophilic substances are discussed briefly.

Dechlorination of chlorocatechols by stable enrichment cultures of anaerobic bacteria

Metabolically stable anaerobic cultures obtained by enrichment with 5-bromovanillin, 5-chlorovanillin, catechin, and phloroglucinol were used to study dechlorination of chlorocatechols. A high degree of specificity in dechlorination was observed, and some chlorocatechols were appreciably more resistant to dechlorination than others: only 3,5-dichlorocatechol, 4,5-dichlorocatechol, 3,4,5-trichlorocatechol, and tetrachlorocatechol were dechlorinated, and not all of them were dechlorinated by the same consortium. 3,5-Dichlorocatechol produced 3-chlorocatechol, 4,5-dichlorocatechol produced 4-chlorocatechol, and 3,4,5-trichlorocatechol produced either 3,5-dichlorocatechol or 3,4-dichlorocatechol; tetrachlorocatechol produced only 3,4,6-trichlorocatechol. Incubation of uncontaminated sediments without additional carbon sources brought about dechlorination of 3,4,5-trichlorocatechol to 3,5-dichlorocatechol. O-demethylation of chloroguaiacols was generally accomplished by enrichment cultures, except that catechin enrichment was unable to O-demethylate tetrachloroguaiacol. None of the enrichments dechlorinated any of the polychlorinated phenols examined. The results suggested that dechlorination was not dependent on enrichment with or growth at the expense of chlorinated compounds and that it would be premature to formulate general rules for the structural dependence of the dechlorination reaction.

Bioavailability of Chlorocatechols in Naturally Contaminated Sediment Samples and of Chloroguaiacols Covalently Bound to C 2 -Guaiacyl Residues

Bacteria in anaerobic enrichment cultures that dechlorinated a range of chlorocatechols were used to examine the stability of endogenous chlorocatechols in a contaminated sediment sample and in interstitial water prepared from it. During incubation of the sediment sample for 450 days with or without added cells, there was a decrease in the concentration of solvent-extractable chlorocatechols but not in that of the total chlorocatechols, including sediment-associated components. In the presence of azide, the decrease in the concentrations of the former was eliminated or substantially decreased. Control experiments in which 3,4,5-trichlorocatechol was added to the sediment suspensions after 130 days showed that its dechlorination was accomplished not only by the added cells but also by the endemic microbial flora. It was concluded that the endogenous chlorocatechols in the sediment were not accessible to microorganisms with dechlorinating activity. On the other hand, microorganisms were apparently responsible for decreasing the solvent extractability of the chlorocatechols, and this effect decreased with increasing length of exposure time. Similar experiments carried out for 70 days with the sediment interstitial water showed that the chlorocatechols that were known to be associated with organic matter were also inaccessible to microbial dechlorination. Experiments with model compounds in which 4,5,6-trichloroguaiacol and tetrachloroguaiacol were covalently linked to C 2 -guaiacyl residues showed that these compounds were resistant to O demethylation or dechlorination during incubation with a culture having these activities. The only effect of microbial action was the quantitative reduction in 12 days of the C′1 keto group to an alcohol which was stable against further transformation for up to 65 days. The results of these experiments are consistent with the existence of chlorocatechols and chloroguaiacols in contaminated sediments and illustrate the cardinal significance of bioavailability in determining their recalcitrance to dechlorination and O demethylation, respectively. It is suggested that bioavailability is an important factor in determining the persistence of xenobiotics in natural ecosystems and that its omission represents a serious limitation in the interpretation of many laboratory experiments directed towards determining the persistence of xenobiotics in aquatic ecosystems.

Role of sulfate concentration in dechlorination of 3,4,5-trichlorocatechol by stable enrichment cultures grown with coumarin and flavanone glycones and aglycones

Metabolically stable anaerobic enrichment cultures have been obtained from sediment samples contaminated with chlorophenolic compounds. Enrichment was carried out with esculin, esculetin, naringin, naringenin, fraxin, quercetin, and acetate in media with two sulfate concentrations. These cultures were used to examine the O-demethylation of 4,5,6-trichloroguaiacol and the dechlorination of 3,4,5-trichlorocatechol. Whereas O-demethylation was observed in all cultures, the occurrence of dechlorination was significantly more restricted. The presence of the carbohydrate moiety in the cultures enriched with the glycones repressed development of populations which were able to carry out dechlorination. Although sulfate at a concentration of 2 g/liter in the primary enrichments blocked the development of populations able to bring about dechlorination, addition of sulfate at this concentration did not inhibit dechlorination in cultures possessing this capability. Different dichlorocatechol isomers were produced under the various conditions, so that in view of the established resistance of some of these to further dechlorination, the ultimate fate of 3,4,5-trichlorocatechol in the natural environment remains partly unresolved. No enrichment culture containing a low sulfate concentration was able to dechlorinate either 2,4,5-trichlorophenol or 2,4,6-trichlorobenzoate.

Transformations of Halogenated Aromatic Aldehydes by Metabolically Stable Anaerobic Enrichment Cultures

Metabolically stable enrichment cultures of anaerobic bacteria obtained by elective enrichment of sediment samples from the Baltic Sea and Gulf of Bothnia have been used to study the oxidation and reduction of the aldehyde group of various halogenated aromatic aldehydes. During the transformation of 5- and 6-chlorovanillin, 6-bromovanillin, 3-chloro-4-hydroxybenzaldehyde, 3,5-dichloro-4-hydroxybenzaldehyde, and 3,5-dibromo-4-hydroxybenzaldehyde, it was shown that synthesis of the corresponding carboxylic acids, which were the principal metabolites, was invariably accompanied by partial reduction of the aldehyde to a hydroxymethyl group in yields of between 3 and 30%. Complete reduction to a methyl group was observed with some of the halogenated vanillins, but to an extremely limited extent with the halogenated 4-hydroxybenzaldehydes. One consortium produced both the hydroxymethyl and methyl compounds from both 5- and 6-chlorovanillin: it was therefore assumed that the methyl compound was the ultimate reduction product. On the basis of the kinetics of formation of the metabolites, it was concluded that the oxidation and reduction reactions were mechanistically related. In addition to these oxidations and reductions, dehalogenation was observed with one of the consortia. In contrast to the transformations of 5- and 6-chlorovanillin, which produced chlorinated methylcatechols, the corresponding compounds were not observed with 5- and 6-bromovanillin: the former was debrominated, forming 4-methylcatechol, whereas the latter produced 6-bromovanillyl alcohol without demethylation. Similarly, although 3-chloro-4-hydroxybenzaldehyde formed the chlorinated carboxylic acid and the benzyl alcohol, the 3-bromo compound was debrominated with formation of 4-hydroxybenzoic acid and, ultimately, phenol. On prolonged incubation, the halogenated carboxylic acids were generally decarboxylated, so that the final products from these substrates were halogenated catechols or phenols. Reductive processes of the type revealed in this study might therefore plausibly occur in the environment during anaerobic transformation of halogenated aromatic aldehydes containing hydroxyl and/or methoxyl groups.

Transformations of chloroguaiacols, chloroveratroles, and chlorocatechols by stable consortia of anaerobic bacteria

Metabolically stable consortia of anaerobic bacteria obtained by enrichment of sediment samples with 3,4,5-trimethoxybenzoate (TMBA), 3,4,5-trihydroxybenzoate (gallate [GA]), or 5-chlorovanillin (CV) were used to study the anaerobic transformation of a series of chloroveratroles, chloroguaiacols, and chlorocatechols used as cosubstrates. Experiments were carried out with growing cultures, and the following pathways were demonstrated for metabolism of the growth substrates: (i) TMBA produced GA, which was further degraded without the formation of aromatic intermediates; (ii) GA formed pyrogallol, which was stable to further transformation; and (iii) CV was degraded by a series of steps involving de-O-methylation, oxidation of the aldehyde group, and decarboxylation to 3-chlorocatechol before ring cleavage. Mono-de-O-methylation of the cosubstrates occurred rapidly in the order 4,5,6-trichloroguaiacol greater than 3,4,5-trichloroguaiacol approximately 3,4,5-trichloroveratrole approximately tetrachloroveratrole greater than tetrachloroguaiacol and was concomitant with degradation of the growth substrates. For the polymethoxy compounds--chloroveratroles, 1,2,3-trichloro-4,5,6-trimethoxybenzene, and 4,5,6-trichlorosyringol--de-O-methylation took place sequentially. The resulting chlorocatechols were stable to further transformation until the cultures had exhausted the growth substrates; selective dechlorination then occurred with the formation of 3,5-dichlorocatechol from 3,4,5-trichlorocatechol and of 3,4,6-trichlorocatechol from tetrachlorocatechol. 2,4,5-, 2,4,6-, and 3,4,5-trichoroanisole and 2,3,4,5-tetrachloroanisole were de-O-methylated, but the resulting chlorophenols were resistant to dechlorination. These results extend those of a previous study with spiked sediment samples and their endogenous microflora and illustrate some of the transformations of chloroguaiacols and chlorocatechols which may be expected to occur in anaerobic sediments.

Bacterial O-methylation of halogen-substituted phenols

Two strains of bacteria capable of carrying out the O-methylation of phenolic compounds, one from the gram-positive genus Rhodococcus and one from the gram-negative genus Acinetobacter, were used to examine the O-methylation of phenols carrying fluoro-, chloro-, and bromo-substituents. Zero-order rates of O-methylation were calculated from data for the chloro- and bromophenols; there was no simple relationship between the rates of reaction and the structure of the substrates, and significant differences were observed in the responses of the two test organisms. For the gram-negative strain, the pattern of substitution was as important as the number of substituents. Hexachlorophene was resistant to O-methylation by both strains, and tetrabromobisphenol-A was O-methylated only by the gram-positive strain. It is suggested that in the natural environment, bacterial O-methylation of phenols carrying electron-attracting substituents might be a significant alternative to biodegradation.

Biotransformations of Chloroguaiacols, Chlorocatechols, and Chloroveratroles in Sediments

The occurrence of trichloro- and tetrachloroguaiacols, -catechols, and -veratroles and their transformation was studied in freshwater and brackish water sediments putatively exposed to bleachery discharge. The samples contained both chloroguaiacols and chlorocatechols, of which >90% could not be removed by simple extraction. The bound concentrations varied and ranged from 550 μg kg of organic C −1 for 3,4,5-trichloroguaiacol to 8,250 μg kg of organic C −1 for tetrachlorocatechol. Chlorinated substrates added to the aqueous phase were rapidly bound to the sediment with K p values between 1.3 and 2.8 ml kg of organic C −1 for the chloroguaiacols and chloroveratroles and 22 to 36 ml kg of organic C −1 for the chlorocatechols. Sediment samples incubated aerobically brought about O-methylation of 4,5,6-trichloroguaiacol to 3,4,5-trichloroveratrole in a yield of ca. 25%. Under anaerobic conditions, however, de-O-methylation of both the chloroguaiacols and chloroveratroles took place with synthesis of the corresponding chlorocatechols. In separate experiments, the chlorocatechols were not completely stable under anaerobic conditions, but their ultimate fate has not yet been resolved. Sediment which had been autoclaved twice at 121°C for 20 min was unable to bring about any of these transformations; we therefore conclude that they were mediated by biological processes. These results emphasize that, in determining the fate of chloroguaiacols and related compounds discharged into the aquatic environment, the cardinal roles of sorption to the sediment phase and of the oxygen tension must be taken into account. We propose a hypothetical guaiacol cycle to accommodate our observations.

Bacterial O-Methylation of Chloroguaiacols: Effect of Substrate Concentration, Cell Density, and Growth Conditions

O-methylation of chloroguaiacols has been examined in a number of gram-positive and gram-negative bacteria to elucidate the effects of substrate concentration, growth conditions, and cell density. Substrate concentrations between 0.1 and 20.0 mg liter −1 were used, and it was found that (i) yields of the O-methylated products were significantly higher at the lowest concentrations and (ii) rates of O-methylation were not linear functions of concentration. With 3,4,5-trichloroguaiacol, the nature of the metabolites also changed with concentration. During growth with a range of substrates, O-methylation of chloroguaiacols also took place. With vanillate, however, de-O-methylation occurred: the chlorocatechol formed from 4,5,6-trichloroguaiacol was successively O-methylated to 3,4,5-trichloroguaiacol and 3,4,5-trichloroveratrole, whereas that produced from 4,5-dichloroguaiacol was degraded without O-methylation. Effective O-methylation in nonproliferating suspensions occurred at cell densities as low as 10 5 cells ml −1 , although both the yields and the rates were lower than in more dense cultures. By using disk assays, it was shown that, compared with their precursors, all of the O-methylated metabolites were virtually nontoxic to the strains examined. It is therefore proposed that O-methylation functions as a detoxification mechanism for cells exposed to chloroguaiacols and chlorophenols. In detail, significant differences were observed in the response of gram-positive and gram-negative cell strains to chloroguaiacols. It is concluded that bacterial O-methylation is to be expected in the natural environment subjected to discharge of chloroguaiacols.